structural PERFORMANCE The Evolution of Fire Safety in Supertall Buildings By James Antell, AIA, P.E.

s structural engineers, our approach to structural fire Asafety in supertall buildings has evolved along with overall fire and life safety goals. Structural systems have progressed from the early steel frame towers of the 1970s to current , . practice incorporating concrete and composite steel/concrete structural elements. This article looks at the relationship between structural engineering practices for tall buildings and how these practices have influenced fire safety strategies for passive and active protection systems in tall buildings over the last 50 years.

Tall buildings have always relied on a combination of passive building safety design. And, automatic sprinkler systems that, at the time, elements and active fire detection, alarm/notification, and suppression used thick-walled schedule 40 pipes with screwed fittings that were systems to control fire and its effects. What has changed is how we cut and threaded on-site, were considered too expensive. use these passive and active elements and the intended results of these The goal of passive elements was to control the spread of fire and to fire safety strategies. This evolution is the result of both advances in maintain the buildings’ structural integrity during occupant evacuation structural and fire safety technology, as well as new safety and security and manual firefighting operations. Structural systems were primarily threats that make the integrity of the building structure and the abil- structural steel frames with steel deck and concrete floor slabs. ity to evacuate occupants in a timely manner of the utmost priority. The 875 N. Michigan Avenue Building in Chicago, formerly the Fifty years ago, the technology available to us to create fire-safe tall John Hancock Center, is a typical example of high-rise construc- buildings was quite basic. Passive protection of the steel structural ele- tion during this period. The building structure consists of a robust ments was provided by sprayed-on cementitious or fiber fireproofing structural steel frame where the perimeter structural system carries materials. These materials were designed to insulate the steel from significant structural load with perimeter diagonal bracing for lat- the heat of a fire and keep the steel below critical temperatures for a eral loads. Although the robust steel structure itself has significant prescribed period (usually 3 hours for the structural frame and 2 hours resistance to heat from a fire by virtue of its high thermal mass, for floors when tested to a standardized fire exposure test). Exit stairs to spray-applied cementitious fireproofing was also applied to the evacuate occupants from the fire area to areas of relative safety within structure to provide the required fire resistance of 3 hours to the the building were provided. Firefighting features such as and structural frame and 2 hours to the floors. firefighting water supplies to facilitate fire department response and Exit stairs, shafts, and mechanical shafts were enclosed with manual suppression of fires at altitude were developed. Still missing fire-rated non-structural drywall or concrete block construction. In from fire protection strategies were reliable fire detection, alarm and those early designs, the building core was typically not a structural voice communications, and automatic suppression by sprinklers. element and did not generally contribute to the structural strength or The idea that tall buildings would someday be targets for a broad fire resistance of the structure itself. Active firefighting systems were range of malicious attacks was not a consideration for the first wave limited to a water supply. In most cases, this included relatively small of tall buildings constructed in the 1960s and 70s in the U.S. on-site tanks and heavily relied upon municipal water supplies pumped to upper levels of the building to serve wet standpipe outlets located in each stair for use by fire department personnel. Those services were The Post WWII Era considered highly reliable, and the need for the building to operate The 1970s witnessed the first, sustained tall building boom in the “off the grid” was not a high priority. United States with iconic, tall buildings constructed in Chicago, New York, and other major cities around the country. Most building codes did not include provisions specifically for high-rise buildings. During Codes “Catch Up” this time, design practices advanced ahead of codes and standards. During the mid-1970s, building regulations in Chicago, New York, and Only after significant experience with high rise design and high-rise throughout the U.S. “caught up” with the design practices of early tall fires did codes and standards adopt specific high-rise provisions. In buildings and mandated specific requirements for tall buildings. The those early days, passive elements were more prevalent than active requirements included most of the passive features incorporated into elements. Fire alarm systems were not considered reliable in fire the earlier designs, including structural fire resistance and enclosed egress

8 STRUCTURE magazine stairs. At the time, automatic sprinklers, active smoke control systems, and emergency voice communication systems were not required by these codes. During the 1980s, the U.S. experienced many signifi- cant fires in high-rise buildings, including hotels and office buildings. However, none of those high-rise fires resulted in the overall failure of the buildings’ structure. In several cases, there was significant localized struc- tural failure where the localized heat of the fire had compromised the steel structural frame with spray-on fireproofing. As a result of those fires, many of the U.S. codes mandated automatic sprinkler protection for all new high-rises and eventually for most existing high-rise buildings. It is widely held that automatic sprinklers are the most effective way of controlling fire size and there- fore controlling heat and smoke from a fire. Sprinkler Petronas Twin Towers and the skyline. system technology had also advanced considerably at this time, reducing costs and improving constructability. stairs, firefighting elevator vestibules, passenger elevators and critical fire alarm, emergency communications, smoke extraction, and fire suppression system risers, protecting them from exposure to a fire in Asia 1990s the occupied portion of the building. The concrete cores also facilitate The next tall building boom occurred in Asia during the 1990s. Two safe “refuge” areas required by local building regulations. iconic projects during this timeframe were the Jin Mao Tower in In the , in addition to the emergency evacuation and the Petronas Twin Towers in Kuala Lumpur, . stairs, firefighting elevator vestibules, and critical life safety risers, the Built contemporaneously, these two projects represent an evolution concrete cores also contain passenger elevators that can be used in in the structural design of tall buildings and the associated fire safety some emergency scenarios to evacuate building occupants from the strategies of these buildings. In both cases, the buildings were designed sky lobby levels. The sky lobbies are designed with limited combustible by U.S. based design and construction teams, using a combination contents so they can act as safe areas for staged building evacuation of local building regulations and international design standards and during emergencies. The sky bridge has the additional benefit of practices. In terms of both structural systems and fire safety, both providing an emergency evacuation route between the towers in case projects were quite advanced for their time. of an emergency affecting the lower portion of one tower. Both designs incorporated poured concrete structural cores with composite perimeter super columns and steel and concrete floor assemblies. The early use of high strength concrete enhanced the 9/11 Brings Changes design efficiency of the core and perimeter columns. This structural As a result of the events of September 11, 2001, the U.S. National system was found to have the advantages of both improved structural Institutes of Science & Technology (NIST) undertook a series of efficiency and improved ease of constructability. With the concrete investigations into all aspects of the design and construction of the core carrying a significant portion of the lateral loads on the building, the struc- tural system had the advantage of allowing the perimeter to be more open to more glass curtain walls and better views for building occupants. The concrete cores provide a highly visit STRUCTUREmag.orgADVERTISEMENT–For Information, Advertiser fire-resistant enclosure in of the towers, separated from the occupied areas of the buildings where fire is likely to occur. All critical life safety egress stairs, firefighting elevators, and critical risers’ power, fire alarm, fire sprinkler, and smoke exhaust are located within the fire-resistant core, protecting them from fire occurring in the perimeter occupied portion of each floor. The concrete core wall also forms a portion of a fire-rated public corridor system surrounding the core and separates occupied office spaces from the common public exits and core elements. In the case of the Jin Mao tower, the con- crete core contains the emergency egress

JUNE 2020 9 World Trade Center towers. The results of Passive fire protection is achieved primar- these investigations, published in the docu- ily by the building’s concrete structure, ment “NIST NCSTAR 1 Final Report on the which acts as a fire-rated enclosure for all Collapse of the World Trade Center Towers,” exit stairs, firefighting elevators, and evacu- includes “areas in current building and fire ation elevators. Critical fire safety system codes, standards, and practices that warrant risers are incorporated into the building’s revision” including “a list of 30 recommen- concrete core. dations for action in the areas of increased Because of the extensive pace of develop- structural integrity, enhanced fire endurance ment in Dubai at the time of construction, of structures, new methods of fire-resistant the building was designed so that it does not design of structures, enhanced active fire rely on municipal water or electric service protection, improved building evacuation, during emergency operations. The fire pro- improved emergency response, improved tection water supply is provided from water procedures and practices, and education tanks of 1- or 2-hour capacity located on all and training.” mechanical floors. Water supplying auto- The NIST recommendations included matic sprinklers is fed by gravity to zones changes to structural design standards to pre- below the tanks, so there is no reliance on vent progressive collapse, as well as changes fire pumps to supply necessary water in an to fire endurance of structures, in-service emergency. performance of fireproofing materials, Finally, evacuation elevators are incorpo- redundancy of active fire protection systems, rated into the design to enable full building and provision for full building evacuation evacuation, if needed, based on a broad range in response to a wide range of emergency of emergency scenarios. Shuttle elevators are scenarios other than fire. operated in “lifeboat” mode to shuttle occu- Many of these recommendations were pants from sky lobbies to grade. adopted by model code groups and were – currently the world’s tallest building. incorporated into their building and fire codes. The 2009International Building Code (IBC) adopted a series The Future: Tower of new requirements specific to buildings more than 420 feet (128 Currently under construction in western , , meters) in height. These requirements include: when complete, will be the world’s tallest building at over 1,000 • Reverting to earlier code requirements mandating a fire meters. Structurally, the building is quite similar to the Burj Khalifa, resistance of the structural frame of 3 hours by removing the using a fully reinforced concrete structure for most of the building, allowance for 2-hour ratings on buildings over 128 meters. incorporating concrete core, shear walls in each wing, columns, and • Design of the structure to eliminate the likelihood of concrete floor slabs. Because the building is residential, it is also highly progressive collapse compartmented by concrete corridor walls and demising walls. Passive • Impact-resistant construction materials for walls enclosing fire protection is achieved primarily by the building’s concrete struc- exits and elevator shafts ture, which acts as a fire-rated enclosure for all exit stairs, firefighting • Minimum bonding strength for spray-applied fireproofing elevators, and evacuation elevators. Critical fire safety system risers materials, so they are less likely to fail in a fire are incorporated into the building’s core. • Redundant water supply sources and risers for automatic One of the distinguishing features of the Jeddah Tower is the use of sprinkler systems highly protected refuge floors every 20 floors, as mandated by local • One additional exit stair for firefighters or elevators designed code. The refuge floors are full floors and, in an emergency, become for emergency occupant evacuation safe areas for occupants’ evacuating the building. Occupants from Many of these enhanced features that were incorporated into high- the 20 floors above each refuge floor can evacuate their zone of the rise buildings on a project-by-project basis were now mandated for building using exit stairs to reach the refuge floor below. Each refuge all buildings over 420 feet. floor is a safe area with minimum connections to other floors and includes independent mechanical systems. During an emergency, they serve as a staging area for elevator evacuation. Shuttle elevators 2000 to the Present are used to evacuate occupants from refuge floors to grade. The design of Burj Khalifa, currently the world’s tallest building, began in 2003 after the events of 9/11, but before the release of the NIST report and the incorporation of the report’s findings into the Conclusion 2009 IBC. The design does incorporate several features that even- Over the past 50 years, our approach to fire safety in tall buildings has tually became part of the 2009 IBC requirements. The design also evolved considerably, incorporating new technologies to provide build- incorporated some relevant aspects of the local code in conjunction ings that are protected from a broad range of threats from with international codes (IBC) and fire safety enhancements. fire, natural disasters, and malicious acts. As we keep building Structurally, the use of a reinforced concrete structure incorporat- taller, our strategies and technologies will continue to evolve.■ ing a concrete core, shear walls in each wing, columns, and concrete floor slabs was dictated by the fact that the building is primarily a James Antell is a Regional Practice Leader for Telgian Engineering & residential occupancy and the shape of the building is three wings. Consulting. Mr. Antell acts as an advisor to design teams and building Because the building is residential, it is highly compartmented by owners regarding compliance with local and international fire safety codes [email protected] concrete corridor walls and demising walls. and standards. ( )

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